The invention relates to a process for mixing and proportioning a plurality of mixing components in which, individually weighed, the mixing components are supplied to a continuous mixing process and the mixture is then removed in a continuously proportioned manner. The invention also extends to an apparatus for performing such a process.
Such processes and apparatuses are primarily used in order to mix several mixing components and supply them in proportioned manner to an extruder for further processing. In general a main mixing component representing the largest proportion by weight, e.g. plastic granular material is mixed with various mixing components having a smaller weight, e.g. dye additives, stabilizers, etc. For such extruders to operate in a satisfactory manner, the devices connected upstream thereof for mixing and proportioning the mixing components must operate continuously on their discharge side.
In this connection it is known from German Offenlegungsschrift No. 22 47 518 to construct at least the feed means for the proportionally smaller mixing components as timed weighing machines, whereas the feed means for the larger mixing component is gravimetrically controlled and continuously proportioned, e.g. as a belt weighing machine.
Removal takes place by means of a discharge screw, which is driven at a given speed, i.e. a volumetrically proportioned removal takes place. The mixing process in the mixer takes place in two stages in a conical screw mixer, the addition of the individual mixing components taking place in a pre-mixing zone and a substantially feed-neutral zone is formed between the pre-mixing zone and a post-mixing zone. Removal from the post-mixing zone takes place by means of a removal device constructed as a proportioning screw, which then supplies a mixture to an extruder. In this known apparatus and process, the individual weighing of the mixing components involves high effort and expenditure and can lead to faults, particularly if difficulty flowing mixing components are used. In addition, the proportioning accuracy of the removal side is not adequate.
It is also known to supply the individual mixing components to the mixer in a continuous manner by means of belt weighing machines. It is already known from German Offenlegungsschrift No. 21 49 242 to supply the main mixing component continuously by means of a belt weighing machine and the other mixing components continuously by means of volumetrically proportioning feed means. All continuous weighing means, particularly belt weighing machines, are extremely costly. In addition, and particularly in the case of belt weighing machines, there is also the problem of their high susceptibility to error when contaminated by dust and the like, so that frequent recalibration is necessary.
German Offenlegungsschrift No. 23 36 104 discloses arranging on a weighing machine a bin for receiving a charge of mixing components to be mixed together and said bin is followed by a conical screw mixer. The individual components are successively supplied, after corresponding weighing to the bin. As a result there is no need for a weight-constant removal behind the conical screw mixer over a period of time.